Free-space loss
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In telecommunication, free-space loss is the signal attenuation that would result if all absorbing, diffracting, obstructing, refracting, scattering, and reflecting influences were sufficiently removed so as to have no effect on its propagation.
Note: Free-space loss is primarily caused by beam divergence, i.e., signal energy spreading over larger areas at increased distances from the source.
The equation for free-space loss is below where <math>\lambda<math> is the signal wavelength, f is the signal frequency, R is the distance or radius of the signal from the transmitter, and c is the speed of light in the signal transmission medium. Note that the units used should be consistent, e.g., <math>\lambda<math> and R in meters, and c in meters/second.
<math>free\ space\ loss = \left ( \frac{4\pi R}{\lambda} \right )^2 = \left ( \frac{4\pi R f}{c} \right )^2<math>
Source: from Federal Standard 1037C and from MIL-STD-188